cmake: Fix patching bug introduced in previous commit

devel/cmake/Portfile

@@ -195,12 +195,6 @@ post-patch {
     # patch PREFIX
     reinplace "s|__PREFIX__|${prefix}|g" ${worksrcpath}/macports.cmake
     reinplace "s|__PREFIX__|${prefix}|g" ${worksrcpath}/Modules/CMakeFindFrameworks.cmake
-
-    # patch Python Version; doing this is OK even when +docs is not
-    # used, because the PYTHON_EXECUTABLE is used only when building
-    # docs -- so, when not building docs its setting makes no difference.
-
-    reinplace "s|__PYTHON_VERSION_WITH_DOT__|${PYTHON_VERSION_WITH_DOT}|g" ${worksrcpath}/macports.cmake
 }
 
 post-destroot {
@@ -283,6 +277,7 @@ variant docs description {Build documentation: HTML and manpages} {
             port:py${PYTHON_VERSION_NO_DOT}-sphinxcontrib-serializinghtml \
             port:py${PYTHON_VERSION_NO_DOT}-sphinxcontrib-websupport
     }
+
 }
 
 variant python27 conflicts python34 python35 python36 requires docs \
@@ -334,4 +329,10 @@ if {[variant_isset docs]} {
         set PYTHON_VERSION_WITH_DOT "3.7"
     }
     set PYTHON_VERSION_NO_DOT [join [split ${PYTHON_VERSION_WITH_DOT} "."] ""]
+
+    post-patch {
+        # patch Python Version
+        reinplace "s|__PYTHON_VERSION_WITH_DOT__|${PYTHON_VERSION_WITH_DOT}|g" ${worksrcpath}/macports.cmake
+    }
+
 }